My present invention relates to a method of fastening a solar cell to a support, especially a surface of a building structure. The invention also relates to a solar cell assembled on a building structure or the like and to a mounting system for a solar cell of the type which has a front glass plate and a rear backing in the form of a polyvinylfluoride foil.
The art does not appear to describe any bonding agent or adhesive which is capable of satisfactorily bonding polyvinylfluoride foils to optional support surfaces of structure and the like, especially concrete structures, in a permanent manner. A number of solar cells do provide, however, backing layers of polyvinylfluoride foils. As a consequence, to mount such solar cells on building structures it has been the practice heretofore to secure the solar cells in a metal frame and then to mount this metal frame on the structure. This approach is relatively expensive.
EP 0 790 370 A discloses a bonding of a solar cell with a front glass plate by bonding a mortar layer which is composed of a finely divided neutral aggregate, cement (especially hydraulic cement) and an aqueous dispersion of a polyacrylic acid derivative in which the suspension serves to provide water for hydrating the hydraulic cement and aggregate mixture. The polyacrylic acid derivative has a logarithmic decrement of the torsional vibration damping, determined by the German Industrial Standard DIN 53445 (August 1986), which is a maximum at a temperature below 0xc2x0 C. This bonding composition or mortar serves to secure the solar cell to a support surface by a rear glass plate which must be provided over the polyvinylfluoride foil or other foil structure of the solar cell. As far as I am aware, there has been no adhesive or other bonding agent described heretofore for securing the polyvinylfluoride foil directly to a support surface.
It is therefore the principal object of the present invention to provide an improved method of bonding the polyvinylfluoride backing foil of a solar cell to a support surface.
Another object of the invention is to provide a method of mounting a solar cell which is less expensive than earlier mounting methods and obviates drawbacks thereof.
A further object of the invention is to provide an improved solar cell assembly.
These objects are attained in accordance with the invention by a method of fastening to a support a solar cell having a front glass plate and rear backing of a polyvinylfluoride foil and which comprises the steps of:
(a) preparing a bonding composition from a hydraulic cement, a finely divided neutral aggregate and an aqueous dispersion of a polyacrylic acid derivative having a logarithmic decrement of torsional vibration damping, determined in accordance with German Industrial Standard DIN 53445 (corresponding to ISO 537-1980), having a maximum at a temperature below 0xc2x0 C.;
(b) applying the bonding composition between the rear backing of polyvinylfluoride foil of the solar cell and a support surface; and
(c) permitting the bonding composition to set, thereby bonding the polyvinylfluoride-foil backing of the solar cell to the support surface.
It is indeed surprising that this kind of mortar can bond not only glass but also polyvinylfluoride to surfaces like metal surfaces and concrete surfaces of a structure satisfying all of the requirements for mounting of solar cells in an exposed state. This was certainly not to be expected since, while both polyvinylfluoride and glass are practically impermeable to water vapor, they have vastly different compositions and structures.
In a preferred embodiment of the invention, the back side of the solar cell, i.e. the polyvinylfluoride foil, may be precoated with a hardened layer of a mortar composition which hardens by combination with water of hydration and under conditions of a water of hydration deficiency. Later with the aid of a fresh bonding mortar composition as described above, the deficiency layer and the polyvinylfluoride foil are bonded to the surface of the support which can be constituted of concrete and which may have a cassette-like recess in which the solar cell is seated for protection from external influences.
Solar cells based upon amorphous silicon have an efficiency which increases with the temperature. In a preferred embodiment, to insulate the solar cell and maintain an elevated temperature, the solar cell is secured in accordance with the invention on a glass foam substrate.
With solar cells of crystalline silicon, which have an efficiency which increases as the temperature is reduced, the solar cell can be secured to a sheet metal plate having a rearwardly bent tongue which enables the plate to be hung on the wall of a structure. The sheet metal plate to which the solar cell is bonded has a forwardly bent lip at an edge engaging below the solar cell. The finely divided neutral aggregate can have a grain size of 0.1 to 1 mm, preferably 0.2 to 0.7 mm. The cement can be a Portland cement.
The polyacrylic acid derivative can be a polyacrylate, especially an acrylic resin or a copolymer of polymers in which acrylonitrile is one of the comonomers. The water content of the dispersion serves as the water of hydration or setting the composition and for completing the hydration if necessary of the layer previously applied to the polyvinylfluoride foil.